X-ray absorption spectroscopy has been used to probe the frozen solution structure of the metal site in Pyrococcus furiosus rubredoxin in the native, iron-containing protein, and in zinc and mercury-substituted proteins. For all samples studied, the spectra have been interpreted in terms of a single shell of coordinated sulfur, with approximately tetrahedral coordination. For the native protein we obtain Fe-S bond-lengths of 2.29 and 2.33 E, for oxidized and reduced proteins, respectively. These values are in excellent agreement with those previously obtained from X-ray crystallography. The metal-substituted rubredoxins possess metal-sulfur bond lengths of 2.34 and 2.54 E, for the zinc and mercury-substituted proteins, respectively.